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The relative importance of two dissimilatory ni- trate reduction pathways, denitrification (DEN) and dissim- ilatory nitrate reduction to ammonium (DNRA), was inves- tigated in intact sediment cores from five different coastal marine field sites (Dorum, Aarhus Bight, Mississippi Delta, Limfjord and Janssand). The vertical distribution of DEN ac- tivity was examined using the acetylene inhibition technique combined with N2O microsensor measurements, whereas NH + production via DNRA was measured with a recently developed gel probe-stable isotope technique. At all field sites, dissimilatory nitrate reduction was clearly dominated by DEN (59-131 % of the total NO 3 reduced) rather than by DNRA, irrespective of the sedimentary inventories of elec- tron donors such as organic carbon, sulfide, and iron. High- est ammonium production via DNRA, accounting for up to 8.9 % of the total NO 3 reduced, was found at a site with very high concentrations of total sulfide and NH + within and be- low the layer in which NO 3 reduction occurred. Sediment from two field sites, one with low and one with high DNRA activity in the core incubations, was also used for slurry in- cubations. Now, in both sediments high DNRA activity was detected accounting for 37-77 % of the total NO 3 reduced. These contradictory results might be explained by enhanced NO 3 availability for DNRA bacteria in the sediment slurries compared to the core-incubated sediments in which diffusion of NO 3 from the water column may only reach DEN bac- teria, but not DNRA bacteria. The true partitioning of dis- similatory nitrate reduction between DNRA and DEN may thus lie in between the values found in whole core (under- estimation of DNRA) and slurry incubations (overestimation of DNRA).

Vertical activity distribution of dissimilatory nitrate reduction in coastal marine sediments